Intravascular Injection in Lumbar Medial Branch Block: A Prospective Evaluation of 1433 Injections : Anesthesia & Analgesia

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Analgesia: Research Report

Intravascular Injection in Lumbar Medial Branch Block: A Prospective Evaluation of 1433 Injections

Lee, Chul Joong MD, PhD*; Kim, Yong Chul MD, PhD; Shin, Jae Hyuck MD; Nahm, Francis Sahngun MD; Lee, Hyo Min MD; Choi, Yun Suk MD; Lee, Sang Chul MD, PhD; Ko, Justin Sang MD*; Kim, Tae Hyeong MD*; Sim, Woo Seok MD, PhD*; Kim, Chung Su MD, PhD*; Cho, Hyun Sung MD, PhD*

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Anesthesia & Analgesia 106(4):p 1274-1278, April 2008. | DOI: 10.1213/ane.0b013e318162c358
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Lumbar zygapophysial joint pain among patients with low back pain has been increasingly recognized (prevalence: 10%–15%).1 Lumbar zygapophysial joint pain cannot be diagnosed on clinical and radiologic findings alone,2,3 and either diagnostic medial branch blocks (MBBs) or direct intraarticular local anesthetic injections are considered necessary to confirm the diagnosis.4

Lumbar MBB, which blocks nociceptive afferents, has gained popularity for the diagnosis and treatment of lumbar zygapophysial joint pain1,5 and for the selection of patients suitable for radiofrequency medial branch neurotomy.1,6 In this context, MBB refers to block of the medial branches of the L1–4 dorsal rami, including the L5 dorsal ramus proper. At L5, it is the dorsal ramus of L5, rather than its medial branch, that is amenable to blockade.7

For an issue as controversial as back pain, concern might be raised as to whether MBB is as accurate as it is presumed to be. The success and validity of MBB depends on an accurate technique, because only small volumes of local anesthetics should be applied at target sites to ensure block specificity.8 Even if a needle is placed at a target site radiologically, anesthetic can spread to other structures and other sources of spinal pain and thereby result in a false-positive block. Moreover, if local anesthetics are partially or fully injected intravascularly, a false-negative block can occur.

The false-negative block for lumbar MBB has been suggested to be closely linked with intravascular injection.9 The potential complications associated with intravascular injection are a burning pain, anaphylactic reactions, etc.10 Therefore, fluoroscopic guidance using the technique recommended by Dreyfuss et al.8 is recommended for lumbar MBBs to ensure success and to avoid confounding factors and complications.11 However, the classical technique of injecting contrast after aspiration testing and then taking spot radiographs frequently misses intravascular uptake due to rapid contrast washout.12

The purpose of this study was to prospectively evaluate the incidence of, and the factors associated with, intravascular injection during lumbar MBB. In addition, the reliabilities of preinjection aspiration and spot radiography for predicting intravascular injection were assessed.


Study approval was obtained from our IRB, and the study involved the prospective examination of 1433 lumbar spinal injection procedures for MBB performed by seven physicians at our pain management center. Informed consent was obtained from all participants. All patients were screened using clinical predictors13 of response to lumbar MBB during a 10-mo period (September 2005 through June 2006) to treat zygapophysial joint pain. Pregnant patients, those with a known allergy to contrast, and those who underwent spinal surgery were excluded.

Electrocardiogram, pulse oximetry, and arterial blood pressure were monitored during each procedure, and periprocedural IV hydration was performed using lactated Ringer’s solution. No intraoperative sedation was administered. The procedure used involved advancing a needle toward the target site under fluoroscopic guidance, according to a previously described method.12 When the needle tip was located at the target site, a careful aspiration test was performed with 1 mL syringe. The term “flashback of blood” is defined as a backflow of blood into the hub of the needle, extension tube, or syringe during preinjection aspiration. Regardless of blood flashback, 0.5 mL of contrast (Iopamidol®, 300 mgI/mL; Bracco s.p.a., Milan, Italy) was injected without needle repositioning and then a spot radiograph was taken. If the contrast showed a serpiginous pattern typical of intravascular uptake and the spot radiograph failed to demonstrate significant contrast accumulation at the target site, the injection was considered intravascular (Fig. 1). After taking the spot radiograph, 0.5 mL of additional contrast was injected under real-time fluoroscopic visualization, again without needle repositioning. If the contrast flowed in a serpiginous pattern and disappeared within seconds, the injection was considered intravascular.

Figure 1.:
Oblique fluoroscopic image demonstrating spread of contrast at the target site (solid arrows) and a typical intravascular contrast flow pattern (open arrows) during a right L3 medial branch block.

In cases of intravascular uptake confirmed by real-time fluoroscopy, the needle was repositioned and the possibility of an intravascular injection was rechecked before administering medications. Injections of contrast after needle reposition were not included in this study.

Data collected for each injection included physician experience, patient age and sex, presence or absence of intravascular uptake on spot and real-time fluoroscopic radiographs, blood flashback, needle gauge, and block spinal level. Regardless of the total number of injections given to a single patient, injections were recorded separately.

The statistical methods used included the χ2 test, the Z test. Probability values of <0.05 were considered statistically significant.


One thousand four hundred thirty-three MBBs were performed in 456 patients (mean age, 62 yr; range, 23–87 yr), and 711 (49.5%) and 722 (50.5%) were performed in left and right sides, respectively.

The overall incidence of intravascular uptake was 6.1% (88/1433), and in male and female patients incidences were 4.6% (15/328) and 6.6% (73/1105), respectively (Z = 1.3468, P = 0.1780). Patient age (categorized in decades) showed no significant correlation with the incidence of intravascular uptake (χ2 = 2.3219, P = 0.8878).

Among 88 cases of documented intravascular injections, blood flashback was noted in 30 (sensitivity = 34.1%; Table 1) and a positive finding for intravascular uptake was noted by spot radiography in 52 cases (sensitivity = 59.1%; Table 2).

Table 1:
Sensitivity, Specificity, and Predictive Values of the Aspiration Test
Table 2:
Sensitivity, Specificity, and Predictive Values of Spot Radiography with Contrast Enhancement

Among the 1345 nonvascular injections confirmed by real-time fluoroscopy, no blood flashback was observed in 1338 (specificity = 99.5%; Table 1) and no positive finding for intravascular uptake by spot radiography in 1342 (specificity = 99.8%; Table 2).

Fifty eight of 1396 cases without blood flashback by the aspiration test were intravascular (false-negative rate = 4.2%), and 36 of 1378 cases without intravascular uptake by spot radiography were intravascular (false-negative rate = 2.6%). The difference between false-negative rates by aspiration testing and by spot radiography was significant (Z = 2.2446, P = 0.0125; Table 1).

The number of cases of L1 MBBs was too small to reach any statistically significant conclusions about the incidence of intravascular injection at this level. Thus, the influence of MBB level on the incidence of intravascular uptake was analyzed, after excluding the MBB cases performed at the L1 level (Table 3). Differences between the incidences of intravascular uptake from L2 to L5 showed no statistical significance (χ2 = 7.9770, P = 0.0924).

Table 3:
Incidence of Intravascular Contrast Uptake During Lumbar Medial Branch Blocks by Level

A 22-gauge needle was used for 98.7% of MBBs. Other gauges were used too infrequently for statistically significant conclusions to be drawn about the incidence of intravascular uptake for individual needle sizes.

Level of physician experience of MBB ranged from 0.5 to 3 yr, with a mean of 1.4 yr. The incidences of intravascular uptake for the seven physicians ranged from 2.9% to 12.7%. Physicians with <1 yr of experience showed a higher incidence of intravascular uptake (9.4%) than physicians with 1 or more years of experience (3.0%). However, this difference failed to reach statistical significance (P = 0.0571).


In this study, the overall incidence of intravascular injection during MBB was found to be 6.1%, which is similar to the result obtained by Dreyfuss et al.8, who noted an 8% incidence of intravascular uptake in a cadaver study. However, their study was limited to 15 subjects and a total of 120 injections. No large study has been performed on the incidence of intravascular injection during MBB.

In the present study, the prediction of intravascular injection by the aspiration test showed a sensitivity of 34.1%. This low level may be ascribed to pressures in the venous system that are too low to produce spontaneous blood flow through the needle and to collapsed vessels due to the negative pressures generated by aspiration.14 The sensitivity of the aspiration test as determined by the present study is less than determined by other studies, including a cervical transforaminal epidural injection study (45.9%)14 and for lumbosacral transforaminal epidural steroid injections (44.7%).15 This may be explained by variations in vessel sizes, whereby a companion vein of the lumbar medial branch is smaller and more collapsible than vessels in the epidural space. In agreement with other studies on spinal injections,14–16 blood flashback was not found to be a reliable predictor of an intravascular injection during MBB. On the other hand, the specificity of the aspiration test was 99.5%, which is similar to that reported for cervical (97.0%)14 and lumbar transforaminal epidural injections (97.9%).15

In the present study, spot radiographs taken immediately after contrast injection showed 59.1% sensitivity, which is considerably higher than flashback, and a significantly lower false-negative rate. However, spot radiographs demonstrated intravascular uptake in only about 40% of those documented by real-time fluoroscopy. If the needle tip is partially placed into a vessel, contrast may be injected both intra- and extravascularly and at the target site. In such cases, intravascular injection may not be noticed on spot radiographs (Fig. 2A). On the other hand, real-time fluoroscopy is likely to be helpful in such cases because it has the advantage of simultaneously showing target accumulation and intravascular contrast uptake (Fig. 2B).

Figure 2.:
Spot radiograph shows spread of contrast at a target site (solid arrows) without intravascular uptake during a left L3 medial branch block (A). On the other hand, real-time fluoroscopy showed a typical serpiginous pattern of intravascular uptake (open arrows) and contrast at the target site (solid arrows) (B).

Some physicians believe that, if there was some detectable contrast accumulation next to the target site with a portion of that contrast carried away with the blood flow, the MBB would likely be diagnostic. However, we believe that nobody could predict the diagnostic value in that case because the diagnostic MBB requires only a small volume and the blood released from the vein could dilute or disperse the local anesthetic.9 In addition, Kaplan et al.9 reported that, when initial venous uptake occurs (despite subsequent lack of venous uptake with needle repositioning), it carries a 50% risk for false-negative results.

Although the role of adjuvants with steroids in providing long-term relief with MBBs has been controversial,5,17–19 some physicians used 0.5 to 2.0 mL of a mixture of local anesthetic with or without steroid for each nerve.5,19,20 During the therapeutic lumbar MBBs performed in such studies, intravascular injection could lead to potential risks because of the use of steroids and more local anesthetics than the diagnostic MBBs.

Interestingly, we noted blood flashback in 7 of the 1345 nonvascular injections confirmed by real-time fluoroscopy. This finding is probably attributable to tissue trauma and/or blood uptake that occurred when the needle was passed through some intravascular structure en route to the target site.14,16 Based on our experience, such a finding usually occurs when the physician has repositioned the needle several times to properly locate it at the target site and, thus, is attributed to the increased likelihood of tissue trauma and passage through intravascular structures.

In three cases, a serpiginous pattern of contrast observed on spot radiographs failed to predict intravascular needle placement, and their subsequent real-time fluoroscopic images showed no vascular flow or contrast washout. In these cases, it is believed that the physician might have falsely judged the serpiginous pattern of contrast spread through tissues around the target site as intravascular injection by spot radiography.

Sullivan et al.16 reported that both degenerative spinal changes and impairment of venous return to the heart are more common in older patients, and suggested that these anatomical and physiological changes probably play a role in increasing the incidence of intravascular injection in older patients during lumbar spinal injections. In contrast to this suggestion, the results of our study, and those of several other studies, show no significant relation between the incidence of intravascular injection and age.14,15

The incidences of incorrect needle placement and subsequent intravascular injection are higher when the procedure is performed by an inexperienced physician.15,21 Our results showed a nonstatistically significant higher incidence of intravascular injection for physicians with less than, versus more than, 1 yr of experience. This may imply that the block technique is easier to learn than other spinal injection techniques, and that experience is a factor only during the first year of training.

The efficacy of lumbar MBB was not evaluated in this study. Moreover, none of the participants experienced adverse symptoms directly related to an intravascular injection.

Concern has been expressed about the increased radiological exposure required when real-time fluoroscopy is performed during block as opposed to aspiration testing or spot radiography. Thus, the use of pulsed imaging mode, fluoroscopic image collimation, the wearing of protective devices, such as a leaded apron, glasses, gloves, thyroid shields, and other types of lead barriers, are recommended to minimize physician exposure.22 Extended tubing for the administering contrast may also be of benefit.

In conclusion, no specific variables directly related to the incidence of intravascular uptake during lumbar MBB were identified. Both the aspiration test and spot radiography frequently missed intravascular contrast uptake during MBB. We strongly recommend that real-time fluoroscopy with contrast enhancement be performed during the block procedure to increase diagnostic and therapeutic values and to avoid complications.


The authors thank Eun Hyung Lee, MD, Ju Yeon Joh, MD, Sun Sook Han MD, Jung Ran Hwang, RN, Ji Eun Lee, RN, and Dong Soon Kim, RN for their valuable contributions to data collection.


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